A widely used technique for growing tubular crystalline bodies from a melt is the edge-defined, film-fed, crystal growth technique (the EFG process). A customary use of the EFG process is to grow hollow crystalline bodies having a polygonal cross-section, such as "octagons" or "nonagons", for solar cell manufacture. The hollow bodies are grown on a seed from a liquid film of feed material which is transported by capillary action from a crucible containing a quantity of molten material, such as a silicon melt, to the top end of a die having the desired cross-sectional shape. A pulling mechanism is employed for drawing the crystalline body away from the die until a desired length is reached, at which time the crystalline body is removed from the apparatus and a new one drawn. The thus grown hollow tube is then subdivided into a plurality of flat substrates or wafers that are used to form photovoltaic solar cells.
The apparatus used in growing hollow polygonally-shaped crystalline bodies of silicon and the like by the EFG method customarily includes a radiation shield mounted to the crucible inside of the EFG die tip, an inner after-heater that is surrounded by the growing crystalline body and an outer after-heater that surrounds the growing crystalline body, as shown by U.S. Pat. Nos. 4,440,728 issued to R. W. Stormont et al, 4,661,324 issued to N. C. Sink et al, 5,106,763 issued to B. R. Bathey et al, 5,098,229 issued to F. U. Meier et al, and 5,102,494 issued to D. S. Harvey et al.
Residual stresses tend to be present in such hollow bodies as a result of non-uniform changes in temperature of the crystalline body during growth, which can result in or promote buckling, non-flat faces, fracture, plastic flow or creep of the hollow body during growth or during subsequent handling and processing, e.g., during laser cutting into solar blanks.
A number of methods have been proposed to reduce the formation of residual stresses when growing sheet crystals. Annealing the sheet crystal has been suggested. It was expected that annealing at a temperature high enough so that stress relaxation could occur, but lower than its melting point, would relieve residual stresses in grown crystals. However, such a process is not effective in crystals with stresses above 20,000 psi, for example. In any event, annealing means an extra step that is added to the growth process, and thus adds unwanted cost.
U.S. Pat. No. 4,158,038, issued to Jewett, proposed that a crystal temperature profile controller be employed which would provide a substantially linear temperature gradient along the length of a crystalline body as the body is progressively pulled from the growth interface, so as to reduce thermal stresses in the crystalline body. Such a controller consists of a heater which is disposed along the pulling axis of the crystal close to but downstream of the melt/growth interface, with the downstream (higher above the interface) end of the controller being at a substantially lower temperature than the upstream (closer to the interface) end of the controller. The predominant heat flow process along the length of the heater is by conduction and radiation so that it exchanges heat with the moving crystal body. Therefore, the controller induces a thermal distribution lengthwise along the crystal body closely corresponding to its own. However, the Jewett et al device was designed to reduce temperature induced stresses along the length of the growing body and not to lateral stresses. Still other efforts have been made to reduce residual stresses in crystalline bodies grown by the EFG process.
In any event prior efforts have not fully solved the problem of relieving thermoelastic stress along the faces of the polygonally shaped hollow bodies. It has been found that in growing crystalline silicon hollow bodies using prior known apparatus, heat transfer at the corners of the faces is greater than at their center. As a result, uneven heat transfer occurs across the face of the crystal which results in thermal stresses that promote buckling or fracture of the hollow body.